Traditionally oil and gas wells have been drilled using a drill bit connected to the lower end of a drill string. As the drill bit penetrates the earth, the drill string is gradually lengthened. A drilling fluid is usually pumped downward through the drill string to the drill bit to flush and wash away cuttings and debris from around the drill bit and to cool and lubricate the drill bit. The cuttings, debris and mud are returned under pump pressure upwardly through the annulus between the drill string and the walls of the well bore. This process requires large volumes of water and results in the generation of large quantities of contaminated drilling mud that must be stored and then transported and disposed. This has obvious disadvantages both economic and environmental.
U.S. Pat. No. 5,093,008 discloses a dewatering process for recovering water from waste drilling fluid. The process includes chemically inducing flocculation in the waste drilling fluid. The drilling fluid is subsequently transferred to a centrifuge where it is separated into solid waste and clear reusable water.
Another problem associated with drilling of well bores is that oxygen from the atmosphere may contaminate the drilling fluid. Oxygen in the drilling fluid is a major disadvantage because the circulation of the fluids in the well bore brings the drilling fluid into contact with the entire interior and exterior surfaces of the drill string and drill bit. Since oxygen in drilling fluids causes corrosion of any metal surface it contacts, this could result in serious damage to very expensive equipment. In addition, in some drilling environments, oxygen could provide an explosive mixture. U.S. Pat. No. 4,350,505 discloses means whereby the oxygen content of the drilling fluid is replaced by nitrogen or exhaust gases. This however only relates to replacing oxygen in fluids and does not reduce the quantity of water required.
An improved method of well bore drilling is termed underbalanced drilling. The term underbalanced drilling refers to drilling operations where the hydrostatic head of the fluid column is lower than the reservoir pressure. The use of air or other gas as the circulating drilling fluid is well known in the industry. Gas drilling results in increased drill bit life and reduced drilling time. However, when oxygen containing gases such as atmospheric air are used, there is a danger of down-hole explosions. Another drawback of drilling using atmospheric air or aerated drilling fluids is the resultant high oxygen level downhole since, as stated above, this is associated with accelerated corrosion of drill parts. Applicant's U.S. Pat. No. 5,663,121 addresses some of these disadvantages. It discloses a method of using exhaust gases from engines at the drill site as the source of gas for underbalanced drilling which provides for a safer and more economic process. It is clearly apparent, however, that the exhaust from various types of engines can be used, for example, from the engines which drive the drill rig, and most preferably from the engines which drive the exhaust gas compressor. The present invention discloses further improvements of the process. It has now been found that the presence in exhaust gases of constituents such as carbon dioxide, oxides of nitrogen and oxides of sulphur can contribute to acidification of the drilling fluid which may enhance corrosion of metal equipment and may also contribute to the deposit of a scale-like substance on down-hole equipment. While the process disclosed in U.S. Pat. No. 5,663,121 can effectively neutralize the ability of the exhaust gases to create acids within the compression system where only small residual amounts of water are involved, there remains a real and unsatisfied need to develop improved methods of preventing or reversing the acidification process that are cost-effective. The present invention is directed to the development of such a method.